Definitions

sillimanite

sillimanite

[sil-uh-muh-nahyt]
or fibrolite

Brown, pale green, or white glassy silicate mineral that often occurs in long, slender, needlelike crystals frequently found in fibrous aggregates. An aluminum silicate, Al2OSiO4, it occurs in high-temperature regionally metamorphosed clay-rich rocks (e.g., schists and gneisses). Sillimanite is found at many localities in France, Madagascar, and the eastern U.S.; a pale sapphire-blue gem variety occurs in the gravels of Sri Lanka.

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Sillimanite also called Bucholzite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779-1864).

Occurrence

Sillimanite is one of three alumino-silicate polymorphs, the other two being andalusite and kyanite. A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed under thin section or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rocks. It is an index mineral indicating high-temperature metamorphism.

Sillimanite has been found in Brandywine Springs, New Castle County, Delaware, USA. By suggestion of the members of the Delaware Mineralogical Society, it was named by the State Legislature in 1977 as the Official Mineral of Delaware

Details

Crystallographic systemOrthorhombic
ColoursWhite, brown and green
LustreSilky if in fibrous form, else vitreous
Hardness7.5
Refractive index 1.641-1.648
Cleavage1
Twinningn/a
PleochroismColourless to pale brown to yellow
Type localityVltava, Susice, Czech Republic.

See also

External links

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